Reading Thumb instructions | Ground Embedded

Read the MOV, LDR, STR, B and BL that appear most in the Thumb instruction set used by Cortex-M, with real disassembly examples.

Cortex-M uses only Thumb-2, a mix of 16-bit and 32-bit instructions. Its advantage is small code size.

Instructions you see often

Instruction	Meaning
`movs r0, #5`	r0 = 5
`ldr r1, [r0]`	r1 = the memory value r0 points to
`ldr r1, [r0, #4]`	r1 = *(r0 + 4)
`str r1, [r0]`	Store r1 in the memory r0 points to
`orr r1, r1, #32`	r1 \|= 0x20
`cmp r0, #10`	Compare r0 with 10, set the flags
`bne label`	Go to label if not equal
`bl func`	Call func (return address goes in lr)
`bx lr`	Return from a function
`push {r4, lr}` / `pop {r4, pc}`	Save registers / restore them and return

Try reading

Three lines that read a value from memory, add 1 and store it back.

ldr   r3, =0x40020014   ; address of GPIOA_ODR
ldr   r2, [r3]          ; read the current value
orr   r2, r2, #0x20     ; set bit 5
str   r2, [r3]          ; write it back

This is GPIOA->ODR |= (1 << 5); in C. At instruction level you can see that if an interrupt changes the same register between the read and the write, that change is lost.